This invention relates to boosted engine control, and more particularly to the coordinated control of the electronic throttle and variable geometry turbocharger (VGT) in boosted stoichiometric spark ignition engines.
As is known the art, with engines that operate with a stoichiometric air-to-fuel ratio, the engine torque response is directly linked to the cylinder airflow response, which in turn is linked with the intake manifold pressure response. Hence, fast response of intake manifold pressure with little overshoot in response to the driver""s accelerator pedal command is essential to ensure adequate torque response in these engines. Fast response assures small turbo-lag, which is an extremely important drivability consideration in turbocharged engines. At the same time, overshoot in the intake manifold pressure is also very undesirable since it may translate into the engine torque overshoot and drivability concerns.
Consider a stoichiometric spark ignition engine configuration where a compressor of a turbocharger drives air to the intake manifold of the engine through a variable electronic throttle, such compressor being driven by a variable geometry turbine (VGT) fed by combustion gases in the exhaust manifold of the engine and controlled by a control signal uvgt produced by the engine control unit (ECU). It is clear that the electronic throttle can respond very fast and has a lot of authority over the intake manifold pressure when such throttle is not fully open. Therefore, the electronic throttle can be controlled by the engine control unit (ECU) to respond to a difference between a measured and desired intake manifold pressure so that the measured intake manifold pressure is driven to the desired pressure quickly and with small overshoot. The error signal that drives the electronic throttle thus has the following form:
ethrottle=(pintakexe2x88x92pintake,desired).
where:
ethrottle is the intake manifold pressure error used in formation of the throttle control signal;
pintake is the measure intake manifold pressure; and
pintake,desired is the desired intake manifold pressure.
When the throttle is close to being fully open, its authority over the intake manifold pressure is much decreased. Hence, the VGT has to be used to affect the intake manifold pressure in this situation. There are three natural choices for VGT feedback control in this situation. The VGT can be controlled either based on the difference between (1) measured and desired intake manifold pressure, or (2) measured and desired mass air flow (measured before the compressor), or (3) measured and desired exhaust manifold pressure.
In accordance with the present invention, an engine control system and method are provided wherein a difference between an actual intake manifold pressure and a desired intake manifold pressure and a difference between an actual exhaust manifold pressure and a desired exhaust gas pressure are combined into a single feedback signal to control the VGT while the difference between the actual intake manifold pressure and the desired intake manifold pressure is used to control throttle position.
More particularly, we have determined that none of all these three choices described above is totally desirable. We have established that the feedback on the intake manifold pressure (i.e., (1) above) does provide fast response with small overshoot. Since the same measurement is used to control both the throttle and the VGT, however, it can be confirmed that robustness properties of such a system to variations in the engine parameters caused by, for example engine aging, are not good.
The feedback on mass air flow (i.e., (2) above) is not particularly desirable since in steady-state, mass air flow (MAF) (as measured by MAF sensor located before the compressor) is equal to the cylinder flow, which in turn is determined by the intake manifold pressure. Since the parameters of the engine change, the desired values of the intake manifold pressure and mass air flow can be rendered inconsistent, i.e., they cannot be achieved simultaneously in steady-state. This may cause an unpredictable deterioration in controller and engine behavior when the controller tries to achieve both of these setpoints (i.e., desired intake manifold pressure and desired mass airflow).
Finally, we have recognized that for fast response of the intake manifold pressure at tip-ins the exhaust manifold must exhibit a significant overshoot (a flare). This flare assists in quickly spinning up the turbocharger and minimizing the turbo-lag that would otherwise be significant in the response. Controlling VGT as to keep the exhaust manifold pressure close to its steady-state set-point (i.e., (3) above) will eliminate the flare and will increase the turbo-lag (i.e., decrease the speed of response).
From these considerations, in accordance with the present invention, error signals comprising: (1) a difference between measured intake manifold pressure and a desired intake manifold pressure; and (2) a difference between measured exhaust manifold pressure and desired exhaust manifold pressure are combined into a single feedback signal, evgt, for the variable geometry turbine, where evgt, is:
evgt=Wxc2x7(pintakexe2x88x92pintake,desired)+(1xe2x88x92W)xc2x7(pexhaustxe2x88x92pexhaust,desired).
The weight w is a variable chosen by the control system designer for the system under consideration. It weights the relative influence of intake and exhaust manifold pressure on evgt. The weight w multiplying the intake manifold pressure error is set between 0 and 1 but close to 1 to provide fast transient response in the intake manifold pressure with small overshoot. The small non-zero weight (1xe2x88x92w) on the exhaust manifold pressure error improves the robustness of the controller to parameter variations in steady-state. We found that the value of w=0.8 results in quite good responses.
The controller (e.g. ECU) that drives the throttle and VGT can now be fully specified. It applies proportional plus integral action to both e(t)throttle and e(t)vgt and adds the results of such actions to the feedforward terms of nominal throttle position signal uthrottle,desired(t) and nominal VGT position signal uvgt,desired(t), respectively. Thus, the control signal fed to the VGT is uvgt(t) and the signal fed to the throttle is uthrottle(t) where:
uthrottle(t)=uthrottle,desired(t)+kp,throttleethrottle(t)+ki,throttlezthrottle(t),
zthrottle(t)=zthrottle(txe2x88x92xcex94T)+ethrottle(t),
uvgt(t)=uvgt,des(t)+kp,vgtevgt(t)+ki,vgtzvgt(t),
zvgt(t)=zvgt(txe2x88x92xcex94T)+evgt(t),
and where xcex94T is the sampling period. The proportional controller gains, kp,throttle and kp,vgt and the integral controller gains ki,throttle and ki,vgt, can be made functions of engine speed and intake manifold pressure demand and can be stored in appropriate look-up tables.